1. Field of the Invention
The present invention relates to a surface-mount type coil component having a mount surface for mounting the same on a printed circuit board or a hybrid IC (HIC).
2. Description of the Related Art
Known coil components mounted on circuits in electronic apparatus such as personal computers and cellular phones include wire-wound type components provided by winding a copper wire around a ferrite core, multi-layer type components provided by forming a coil conductor pattern on a surface of a magnetic sheet made of ferrite and stacking such magnetic sheets one over another, and thin-film type components provided by forming insulation films and coil conductors constituted by metal thin films alternately using thin film forming techniques. Multi-layer type and thin-film type coil components can be easily provided with a small size.
Known coil components include common mode filters for suppressing a common mode current which can cause electromagnetic interference in a balanced transmission system. Patent Document 1 discloses a common mode choke coil (common mode filter) provided by stacking a first insulator layer, two lead-out electrodes, a second insulator layer, a first coil conductor, a third insulator layer, a second coil conductor, and a fourth insulator layer in the order listed on a surface of a magnetic substrate. One of the two lead-out electrodes is electrically connected to the first coil conductor through a via hole provided on the second insulator layer, and the other electrode is electrically connected to the second coil conductor through via holes provided on the second and third insulator layers.
To decrease the direct-current resistance of the common mode filter and to improve electrical characteristics of the same such as impedance characteristics and transmission characteristics in a differential mode, the coil conductors of the filter must be formed with a great width. However, when the coil conductors are formed with a greater width, a greater electrostatic capacity (floating capacity) will be generated between the two coil conductors. In a differential mode, an electrostatic capacity is a parasite in parallel with the inductance of a coil conductor. Therefore, when a relatively great electrostatic capacity is generated at the common mode filter, the electrostatic capacity constitutes the dominant impedance of the filter in high frequency bands.
In order to suppress any increase in the electrostatic capacity while forming the coil conductors with a greater width, the distance between the two coil conductors must be made greater (e.g., 20 μm or more). The thickness of the insulator layer between the two coil conductors must be increased to increase the distance between the two coil conductors. When the thickness of the insulator layer between the two coil conductors of the common mode filter disclosed in Patent Document 1 is increased, the insulator layer between the two coil conductors must be formed with a thickness greater than that of the insulator layer between the lead-out electrodes and the first coil conductor.
In order to provide the insulator layer between the two coil conductors with a great thickness, for example, the insulator layer must be provided by performing a plurality of forming steps consecutively. When the insulator layer is formed by performing a plurality of steps consecutively, the number of manufacturing steps increases, which results in an increase in manufacturing time. In order to form the insulator layer between the two coil conductors at a time, for example, it is necessary to form the insulator layer between the two coil conductors from a material different from the material of the insulator layer between the lead-out electrodes and the first coil conductor. When the two insulator layers are formed from different materials, for example, different apparatus must be used to form the insulator layer between the two coil conductors and the insulator layer between the lead-out electrodes and the first coil conductor. A problem therefore arises in that an increase in the distance between the two coil conductors results in an increase in the manufacturing cost of the common mode filter.
Patent Document 2 discloses a thin-film common mode filter provided by stacking a first insulator layer, a lower lead conductor, a second insulator layer, a lower coil conductor, a third insulator layer, an upper coil conductor, a fourth insulator layer, an upper lead conductor, and a fifth insulator layer in the order listed, on an insulated magnetic substrate. The above-described problem occurs also in the thin-film common mode filter disclosed in Patent Document 2.
There is demand for further reduction in the direct-current resistance of common mode filters. Direct-current resistance is the dominant common mode impedance in low frequency bands. It is desirable for common mode filters to have low common mode impedance in low frequency bands. Further, there is demand for reduction in power consumption in products such as cellular phones. A common mode filter having a low direct-current resistance allows a reduction in the power consumption of an electronic apparatus employing the common mode filter. It is therefore desirable for a common mode filter to have a low direct-current resistance.
Patent Document 1: Japanese Patent No. 3601619
Patent Document 2: JP-A-2005-159223